1. Field of the Invention
The present invention relates to a plasma processing method, a method for manufacturing a semiconductor device and a semiconductor device. Particularly, it relates to a method for manufacturing a semiconductor device based on a plasma CVD method, which can suitably provide a crystal or non-singlecrystal functional deposition film useful for a semiconductor device serving as an electrophotographic photosensitive member, a line sensor for image input, an image pickup device, a photovoltaic device or the like, and a semiconductor device manufactured according to the same method.
2. Related Background Art
As a vacuum process used to form a semiconductor device, an electrophotographic photosensitive member, a line sensor for image input, an image pickup device, a photovoltaic device, or other electronic or optical devices, many types of methods have been known, such as vacuum evaporation, sputtering, ion plating, plasma etching, and apparatus for the methods have been put into practical use.
For example, a plasma CVD method, which provides a thin deposition film formed on a substrate by using a direct-current or high-frequency glow discharge to decompose a source gas, has been put into practical use as a suitable measure for forming a deposition film. The plasma CVD method is used for forming a deposition film of amorphous silicon hydride (referred to as an a-Si: H, hereinafter), for example, and various kinds of apparatus used for the method have been proposed.
Such apparatus and method for forming a deposition film will be generally described below.
FIG. 5 is a diagram schematically showing an example of an apparatus for forming a deposition film based on the plasma CVD method (abbreviated as RF-PCVD, hereinafter) using a high frequency power of the RF band, specifically, an apparatus for forming an electrophotographic photosensitive member. The forming apparatus shown in FIG. 5 is arranged as described below.
The apparatus generally comprises a deposition unit 3100, a source gas supply unit 3200, and an exhaust unit (not shown) for reducing pressure in a reactor 3110. The reactor 3110 in the deposition unit 3100 has a cylindrical substrate 3112, a heater 3113 for heating the substrate and a source gas introducing pipe 3114 installed therein. A high frequency matching box 3115 is connected to an electrode 3111 which is part of the reactor 3110. The electrode 3111 is insulated from ground potential by an insulator 3120, and a high frequency voltage can be applied between the electrode and the cylindrical substrate 3112 which is kept at the ground potential and also serves as an anode electrode.
The source gas supply unit 3200 comprises gas cylinders 3221 to 3225 for containing a source gas, such as SiH4, H2, CH4, B2H6 or PH3, valves 3231 to 3235, 3241 to 3245 and 3251 to 3255, and mass flow controllers 3211 to 3215. The source gas cylinders are connected to the gas introducing pipe 3114 in the reactor 3110 via the valve 3260.
The apparatus can be used to form the deposition film as described below, for example.
First, the cylindrical substrate 3112 is installed in the reactor 3110, and gas in the reactor 3110 is exhausted by the exhaust unit (not shown, a vacuum pump, for example). Then, the temperature of the cylindrical substrate 3112 is controlled with the heater 3113 for heating the substrate to be kept at a predetermined temperature from 200 to 350 degrees Celsius.
In order to supply the source gas for forming the deposition film into the reactor 3110, it is first checked that the gas cylinder valves 3231 to 3235 and a leak valve 3117 of the rector container are closed, and that the inlet valves 3241 to 3245, the outlet valves 3251 to 3255 and the auxiliary valve 3260 are opened Then, a main valve 3118 is opened to exhaust gas from the reactor 3110 and a gas pipe 3116.
When a vacuum gage 3119 indicates about 0.1 Pa or lower, the auxiliary valve 3260 and the outlet valves 3251 to 3255 are closed.
Then, the valves 3231 to 3235 are opened for supplying the gasses from the gas cylinders 3221 to 3225, and the pressures of the gasses are controlled at 0.2 MPa by their respective pressure regulators 3261 to 3265. Then, the inlet valves 3241 to 3245 are gradually opened to supply the gasses to the mass flow controllers 3211 to 3215.
After the preparation for the film deposition has been completed in this way, layers are formed according to the following procedure.
When the temperature of the cylindrical substrate 3112 reaches a predetermined temperature, required one of the outlet valves 3251 to 3255 and the auxiliary valve 3260 are gradually opened to introduce a predetermined gas into the reactor 3110 from one of the gas cylinders 3221 to 3225 through the gas introducing pipe 3114. Then, the source gases are regulated to have a predetermined flow rate with the mass flow controllers 3211 to 3215. At the same time, opening of the main valve 3118 is adjusted with the aid of the vacuum gage 3119 so that the pressure in the reactor 3110 is at a predetermined value. When the internal pressure is stabilized, an RF power supply (not shown) with a frequency of 13.56 MHz is set at a predetermined power, and the RF power is introduced into the reactor 3110 through the high frequency matching box 3115 and the cathode 3111 to produce a glow discharge between the cathode and the cylindrical substrate 3112 serving as an anode. The source gas introduced into the reactor is decomposed by discharge energy, and a predetermined deposition film primarily made of silicon is formed on the cylindrical substrate 3112. After the deposition film having a predetermined thickness is formed, the supply of the RF power is stopped, the outlet valve is closed to stop the supply of the gas into the reactor, and thus, the forming of the deposition film is completed.
By repeating the same operation for several times, a desired multilayered photoreception layer can be formed.
It goes without saying that, when forming each layer, the outlet valves other than one for the gas required for the layer are closed. Furthermore, in order to prevent the respective gases from remaining in the reactor 3110 or pipes from the outlet valves 3251 to 3255 to the reactor 3110, the operation of exhausting gas in the system to a high vacuum state by closing the outlet valves 3251 to 3255, opening the auxiliary valve 3260 and fully opening the main valve 3118 is performed as required.
In addition, to assure uniform film formation, it is effective that the cylindrical substrate 3112 is rotated at a predetermined speed with a driving device (not shown) during forming the layer.
In addition, the gas species and the valve operations described above may be, of course, modified according to a condition for forming the respective layers.
Besides such an RF plasma CVD method, a plasma CVD method using a high frequency power in the VHF band (abbreviated as “VHF-PCVD”, hereinafter) has attracted attention, and various developments of film deposition using the method have been actively made.
The VHF-PCVD method can provide a high film deposition speed and high-quality deposition film, and thus is expected to realize the cost reduction and quality improvement of the product at the same time. For example, in Japanese Patent Application Laid-Open No. 6-287760 (corresponding to U.S. Pat. No. 5,534,070), there are disclosed an apparatus and method for forming an a-Si film used as an electrophotographic photosensitive member. Besides, an apparatus for forming a deposition film that can form a plurality of photoreception members for electrophotography and has a high productivity as shown in FIGS. 6A and 6B has been developed.
FIG. 6A is a schematic longitudinal section of an apparatus for forming a deposition film, and FIG. 6B is a schematic cross-sectional view of the apparatus.
A reactor 4111 has an exhaust pipe 4112 integrally formed on its side, and the other end of the exhaust pipe 4112 is connected to an exhaust unit (not shown). Six cylindrical substrates 4113, on which a deposition film is to be formed, are arranged surrounding an electrode 4114. Each of the cylindrical substrates 4113 is held on a rotation shaft 4121 and adapted to be heated by a heat generator 4120. When a motor 4123 is driven, the rotation shaft 4121 is rotated by the motor via a reduction gear 4122, and the cylindrical substrate 4113 is rotated about a longitudinal center axis thereof.
A source gas is supplied to the reactor 4111 from source gas supply means 4118. The VHF power is supplied to the reactor 4111 from a VHF power supply 4116 through a matching box 4115 and then an electrode 4114. At this time, the cylindrical substrates 4113 kept at the ground potential via the rotation shafts 4121 each serve as an anode electrode.
Such an apparatus can be used to form a deposition film generally according to the following procedure.
First, the cylindrical substrates 4113 are installed in the reactor 4111, and gas in the reactor 4111 is exhausted through the exhaust pipe 4112 by the exhaust unit, not shown. Then, the cylindrical substrates 4113 are heated and controlled with the heat generator 4120 at a predetermined temperature from 200 to 300 degrees Celsius.
When the temperature of the cylindrical substrates 4113 reaches a predetermined temperature, a source gas is introduced into the reactor 4111 via the source gas supply means 4118. Once it is confirmed that the flow rate of the source gas is at a predetermined flow rate and the pressure in the reactor 4111 is stabilized, a predetermined VHF power is supplied to the electrode 4114 from the high frequency power supply 4116 through the matching box 4115. In this way, the VHF power is introduced into the reactor 4111, a glow discharge is produced in the reactor 4111, and thus, the source gas is excited and dissociated to form a deposition film on the cylindrical substrates 4113.
After the deposition film having a desired thickness is formed, the supply of the VHF power is stopped, and then the supply of the source gas is stopped to complete the forming of the deposition film. By repeating the same operation for several times, a desired multilayered photoreception layer can be formed.
During forming the deposition film, the cylindrical substrate 4113 is rotated at a predetermined speed with the motor 4123 via the rotation shaft 4121 to form the deposition film that is uniform over the whole surface of the cylindrical substrate.
In Japanese Patent Application Laid-Open No. 62-188783, there is disclosed a method for manufacturing an electrostatic latent image carrier, in which a modulation frequency power of a low frequency alternating current (20 Hz to 1 MHz) and a high frequency alternating current (1 MHz to 100 GHz) superimposed on each other is supplied to an electrode to form and stack amorphous semiconductor layers on a substrate, and thus, any heater is not required and the film deposition speed is improved.
In Japanese Patent Application Laid-Open No. 7-130719 (corresponding to U.S. Pat. No. 5,698,062), there is disclosed a technique of a plasma processing apparatus in which a synthesized high frequency power of at least two frequencies synthesized is applied to an electrode, and the ratio of the frequencies to be synthesized can be adjusted to change the composition ratio of reactive ions in a reactive gas plasma, whereby the precision of machining is enhanced.
In Japanese Patent Application Laid-Open No. 7-74159, there are disclosed plasma processing apparatus and method in which a high frequency power of 60 MHz and a low frequency power of 400 kHz are superimposed on each other to be supplied to an electrode on the side of a substrate, and a self-bias voltage is controlled by changing the value of the low frequency power, thereby increasing an etching rate and reducing particle generation.
The above-described methods and apparatus can realize good plasma processing and deposition film formation. However, the demands of the market on such products are being increased day by day, and there is a need for a method for plasma processing and deposition film formation that can produce a product with higher quality.
For example, in the case of an electrophotographic photosensitive member, there is a high demand for improvements in the process speed, miniaturization, cost reduction, etc., of electrophotographic apparatus. Consequently, it is desirable to enhance electrophotographic photosensitive member properties specifically reflecting such demands as improved chargeability and photosensitivity while increasing the percent of manufacturing non-defectives. In addition, in digital electrophotographic apparatus and color electrophotographic apparatus the widespread use of which is noticeable in recent years, copies are frequently taken not only of letters or documents, but also of photographs, pictures, design drawings, etc., hence reduced optical memory is increasingly demanded. Further, in order to lower unevenness in image density, it is required that a film uniform both in thickness and quality is formed on a large area substrate.
While aiming for such improvements in the properties of photosensitive members, optimizing layer forming conditions and layer constitution is being tried. Besides that, there is a need for making improvements in the process and apparatus for producing electrophotographic photosensitive members.
In such circumstances, there is room still left for improvements in the plasma processing method, and the process and apparatus for producing electrophotographic photosensitive members.
As described previously, by performing a vacuum processing by using a high frequency power with a frequency in or near the VHF band to produce plasma, a speed and characteristics of the vacuum processing can be improved. However, if a high frequency power in such a frequency band is used, the wavelength of the high frequency power in the vacuum processing container is substantially the same as the length of the vacuum processing container, high frequency power electrode, substrate, substrate holder or the like, and thus, a standing wave of the high frequency power is formed in the vacuum processing container. The standing wave causes spatial variations of intensity of electric power in the vacuum processing container, thereby varying plasma characteristics. As a result, while the thickness of the deposition film is substantially uniform in a plane of the substrate, the quality of the film is not uniform. In the case of a large-area substrate to be processed, such as the electrophotographic photosensitive member, the nonuniform quality of the film results in a characteristics unevenness, and it is difficult to provide uniform vacuum processing characteristics over a large area.
In the case of a thick device, such as the electrophotographic photosensitive member, a characteristics distribution of the substrate in a plane thereof varies according to the thickness since the state of plasma is changed in the course of film deposition. Accordingly, the film quality may become nonuniform, or be changed in the thickness direction.
Such an unevenness becomes a significant problem when forming a crystal or non-singlecrystal functional deposition film used for a photovoltaic device, line sensor for image input or image pickup device, as well as the electrophotographic photosensitive member. In addition, also in a plasma processing, such as dry etching or sputtering, when a discharge frequency is increased, the same processing unevenness appears, which would become a significant problem if no measures are taken.
In order to solve such a problem, it is considered that a plurality of different high frequency powers are simultaneously supplied into the reactor. This results in a plurality of standing waves with different wavelengths associated with the respective frequencies. Since high frequency powers are simultaneously supplied, a plurality of standing waves are synthesized, and no distinct standing waves are formed.
According to this consideration, whatever values a plurality of different high frequency powers have, they may exhibit an effect of suppressing the standing waves. For example, in Japanese Patent Application Laid-Open No. 60-160620 (corresponding to U.S. Pat. No. 4,579,618), there is disclosed a structure in which a high frequency power equal to or higher than 10 MHz and a high frequency power equal to or lower than 1 MHz are supplied to one and the same electrode. And in Japanese Patent Application Laid-Open No. 9-321031 (corresponding to U.S. Pat. No. 5,891,252), there is disclosed a structure in which a first high frequency power in the UHF band (equal to or higher than 300 MHz and equal to or lower than 1 GHz) and a second high frequency power with a frequency twice or more as high as that of the first high frequency power are superimposed on each other.
These techniques were expected to suppress the standing waves of the high frequency powers in the reactor and improve the vacuum processing uniformity.
However, in experiments concerning the uniformity carried out by the inventors with the above-described techniques, the uniformity of the vacuum processing characteristics was improved to a certain level, but the uniformity could not attain the level required recently. That is, it was proved that, even with a power supply method that should provide uniform electric field intensity, a certain level of unevenness remains in a practical vacuum processing.